It is known that rubber compositions using, as the basis ingredient, a nitrile-containing highly saturated copolymer rubber obtained by hydrogenating the conjugated diene portions of an unsaturated nitrile-conjugated diene copolymer can yield vulcanized products having excellent resistance to Freon gas, and they are hence being used in various applications. For example, it has been reported that a rubber composition obtained by incorporating a specific amount of carbon black into a nitrile-containing highly saturated copolymer rubber can be used for the formation of O-rings for the compressors of refrigerating machines (Japanese Patent Laid-Open No. 223838/'85), a rubber composition obtained by incorporating an organic peroxide into a nitrile-containing highly saturated copolymer rubber having a Mooney viscosity of not less than 100 can yield a vulcanized product having excellent resistance to fluorocarbon refrigerants (Japanese Patent Laid-Open No. 65369/'93 ), a rubber composition obtained by incorporating carbon black and white carbon having a certain specific surface area into a nitrile-containing highly saturated copolymer rubber can yield a vulcanized product having improved refrigerant resistance (Japanese Patent Laid-Open No. 118447/'95).
However, in recent years, it is desired thatseals and other automobile parts should have a wide service temperature range extending from low to high temperatures. From this point of view, it has become necessary to improve the aforesaid rubber compositions containing a nitrile-containing highly saturated copolymer rubber because the vulcanized products thereof have excellent heat resistance, weather resistance and oil resistance, but are somewhat inferior in low-temperature resistance. In answer to this request, the present applicant has reported a nitrile-containing highly saturated copolymer rubber composition which can yield a vulcanized rubber having improved low-temperature resistance (Japanese Patent Laid-Open No. 95242/'88).
However, it has been found that, when the vulcanized product of this nitrile-containing highly saturated copolymer rubber composition having improved low-temperature resistance is used as a sealing material, particularly as a sealing material for Freons, the following new problems arise. That is, it is difficult to inhibit the permeation of Freon gas; its high degree of swelling with refrigerating machine oil causes a reduction in sealing properties; and minute cracks are produced in the rubber surface owing to foaming caused by the rapid vaporization of Freon gas in the seal surface. Consequently, there is a need for further improvements on it.
An object of the present invention is to provide a vulcanizable rubber composition which can yield a vulcanized product exhibiting excellent low-temperature resistance, sealing properties and resistance to Freon gas permeation and having resistance to the production of cracks in the rubber surface due to the foaming of Freon gas.
Another object of the present invention is to provide a seal for Freons which has excellent low-temperature resistance.